Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel by pixel manner into an image recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the receiver member to yield the desired image. In one process, known as drop-on-demand inkjet printing, individual droplets are ejected as needed on to the recording medium to form the desired image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation using heated actuators. With regard to heated actuators, a heater placed at a convenient location within the nozzle or at the nozzle opening heats ink in selected nozzles and causes a drop to be ejected to the recording medium in those nozzle selected in accordance with image data. With respect to piezo electric actuators, piezoelectric material is used in conjunction with each nozzle and this material possesses the property such that an electrical field when applied thereto induces mechanical stresses therein causing a drop to be selectively ejected from the nozzle selected for actuation. The image data provided as signals to the printhead determines which of the nozzles are to be selected for ejection of a respective drop from each nozzle at a particular pixel location on a receiver sheet. Some drop-on-demand inkjet printers described in the patent literature use both piezoelectric actuators and heated actuators.
In another process, known as continuous inkjet printing, a continuous stream of droplets is discharged from each nozzle and deflected in an imagewise controlled manner onto respective pixel locations on the surface of the recording member, while some droplets are selectively caught and prevented from reaching the recording member. Inkjet printers have found broad applications across markets ranging from the desktop document and pictorial imaging to short run printing and industrial labeling.
A typical inkjet printer reproduces an image by ejecting small drops of ink from the printhead containing an array of spaced apart nozzles, and the ink drops land on a receiver medium (typically paper, coated paper, etc.) at selected pixel locations to form round ink dots. Normally, the drops are deposited with their respective dot centers on a rectilinear grid, i.e., a raster, with equal spacing in the horizontal and vertical directions. The inkjet printers may have the capability to either produce only dots of the same size or of variable size. Inkjet printers with the latter capability are referred to as (multitone) or gray scale inkjet printers because they can produce multiple density tones at each selected pixel location on the page.
Inkjet printers may also be distinguished as being either pagewidth printers or swath printers. Examples of pagewidth printers are described in U.S. Pat. Nos. 6,364,451 B1 and 6,454,378 B1. As noted in these patents, the term “pagewidth printhead” refers to a printhead having a printing zone that prints one line at a time on a page, the line being parallel either to a longer edge or a shorter edge of the page. The line is printed as a whole as the page moves past the printhead and the printhead is stationary, i.e. it does not raster or traverse the page. These printheads are characterized by having a very large number of nozzles. The referenced U.S. patents disclose that should any of the nozzles of one printhead be defective the printer may include a second printhead that is provided so that selected nozzles of the second printhead substitute for defective nozzles of the primary printhead.
Today the fabrication of pagewidth inkjet printheads is relatively complex and they have not gained a broad following. In addition there are problems associated with high-resolution printing in that simultaneous placement of ink drops adjacent to each other can create coalescence of the drops resulting in an image of relatively poor quality.
Swath printers on the other hand are quite popular and relatively inexpensive as they involve significantly fewer numbers of nozzles on the printhead. In addition in using swath printing and multiple passes to print an area during each pass, dot placement may be made selectively so that adjacent drops are not deposited simultaneously or substantially simultaneously on the receiver member. There are many techniques present in the prior art that described methods of increasing the time delay between printing adjacent dots using methods referred to as “interlacing”, “print masking”, or “multipass printing.” There are also techniques present in the prior art for reducing one-dimensional periodic artifacts or “bandings.” This is achieved by advancing the paper by an increment less than the printhead width, so that successive passes or swaths of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. The term “print masking” generally means printing subsets of the image pixels in multiple passes of the printhead relative to a receiver medium. In swath printing a printhead, having a plurality of nozzles arranged in a row, is traversed across a page to be printed. The traversal is such as to be perpendicular to the direction of arrangement of the row of nozzles.
With reference to commonly assigned U.S. Pat. No. 6,464,330 B1 filed in the names of Miller et al., an example of a printhead used in a swath printer is illustrated. The disclosure in this patent is incorporated herein by reference thereto. With reference to FIG. 1, printhead 31 for each color of ink to be printed includes in this embodiment two printhead segments or modules or nozzle banks 39A and 39B. Each printhead nozzle bank includes two staggered rows of nozzles and the nozzles in each row of nozzles have a spacing of {fraction (1/150)} inches between adjacent nozzles in the row. However, due to the presence of staggering there is a nominal nozzle pitch spacing, P, in each printhead nozzle bank of {fraction (1/300)} inches as indicated in the figure. The nozzles on the second nozzle bank 39B are similar to that on the first nozzle bank 39A and the nozzle banks are arranged to continue the nozzle spacing for the printhead of {fraction (1/300)} inches spacing between nozzles. The printhead nozzle banks may each also be referred to as a “nozzle module” because they are individually assembled into a supporting structure to form the printhead for printing a particular color. Each nozzle bank may also be referred to as a pen, segment or a module. Hereinafter, they will be referred to as a nozzle bank. It will be understood that for a printer having six different color inks, six printheads similar to that described for printhead 31 may be provided. The six different color printheads are arranged on a carriage that is traversed across the receiver sheet for a print pass. The nozzles in each of the six color printheads, are actuated to print with ink in their respective colors in accordance with image instructions received from a controller or image processor. Each printhead, would in the example of the subject printer, have two printhead nozzle banks.
The printhead nozzle banks used in inkjet printers can suffer from variations in the manufacturing process that cause the drop size ejected by one nozzle in a nozzle bank to be different from the drop size ejected by another nozzle of that nozzle bank. If this variation in drop size is sufficiently large and of a certain distribution unacceptable banding in printed images can result.
Consider a first hypothetical example in which, because of a manufacturing related processing artifact, there is a drop size variation from one end of a 100-nozzle 1 inch printhead nozzle bank to the other end and the drop size varies linearly between the two extremes. The exemplary printhead nozzle bank is illustrated in FIG. 2. In FIG. 3 there is illustrated a graph showing the drop size variation from nozzle No. 1 to nozzle No. 100. If this printhead nozzle bank was the only one used in printing and used in a 1-pass mode a flat field 2 inch by 2 inch image would appear as illustrated in FIG. 4. The printed result shown in FIG. 4 features for the first vertical inch of the image a gradually increasing density distribution. For the second vertical inch of the image this is repeated, thereby providing an abrupt change in density between the end of the first vertical inch and the beginning of the second vertical inch. In FIG. 5 there is illustrated a graph showing the relationship between the optical density and raster row of print and illustrating quite clearly the abrupt change or discontinuity in density between the 100th row and the 101st row of the image. This abrupt change in density will be noted as an unacceptable banding in a typical image at every one hundred lines of print. Increasing the number of passes decreases the amplitude of this banding and doubles the frequency at the price of lower productivity. Consider the response if we print in a two-pass mode the graphical representation of density of which is illustrated in FIG. 6.
In a printer system with two printhead nozzle banks that are assembled to form a single printhead, as illustrated in FIG. 1, for printing a single color the same problem can arise if no consideration is given as to how printhead nozzle banks are combined.